Torx bits are designed to prevent the mangling of screw heads, with Allens you need to be a bit careful, and flat-blade screwdrivers are a last resort. Beware that some compact Macs have a hidden screw in the battery compartment that you need to remove. The SE doesn't have one, but the Plus, I believe, does.
Lots of people have offered Mac-cracking solutions in UseNet and mailing lists. Here, in no particular order, are some of them . . . .
"I have an original Mac Plus case-cracking kit. It is not rocket science stuff. The 'cracker' is simply a spring-loaded woodworker's gluing clamp. Mine is marked "Pony 3201 Made in USA", but I'm sure a trip to any decent hardware or hobbyist shop would net you a similar one. The jaws (closed) measure 3/4 inch (19 mm) wide and 1/16 inch (1.6 mm) thick. Just insert the jaws and squeeze. As far as the screwdriver goes, my kit came with a bent T-handled hex wrench, not a Torx. Its size is marked 3/32 inch or about 2.38 mm. The shank is 9 inches (about 229 mm) long. A search of hardware stores should turn one up. Obviously, it would be a simple matter to buy an appropriately sized allen (or Torx) wrench, cut off the bent end, insert in a small diameter tube about 14 inches long, and braze or solder it secure, and form a handle to make your own. Or find a source of 3/32 inch allen wrench stock about 15 inches long and bend a T-handle."
"I'm a little puzzled at all the talk about how hard it is to get a T-15 screwdriver. Go to most any auto parts store and ask for an S-K brand T-15 screwdriver and they will sell you one for about $9. It's a bit steep but it is the ideal tool for the Macs with screws recessed in the handle on top of the case. The S-K blade is 6 inches long and easily reaches in to get those two screws. It also has a nice handle to get good torque. If you don't want to spend as much money and have a "Parts Plus" Western Auto store nearby, they had a box of cheapo screwdrivers for 89 cents. The blade is a little shorter but can easily be modified by grinding off a little of the handle until the blade extends far enough into the Mac handle."
"I've read a few letters about the difficulty in getting access to the Torx screws to open up a one-piece Mac. I did it for less than $2. Go to a hardware store and buy two screwdrivers. One for the Torx screws, and one regular slotted screwdriver. Don't pay more than 89 cents for either. Take the Torx screwdriver and put it in a vice and smash the handle (plastic or wood) leaving just the steel shank. Using a hacksaw, file a slot in the end of the shank. If it's a hardened shank, use a torch or gas stove to soften it. Drive the slotted screwdriver into the slot you have just made and 'presto' - you have a Torx extension screwdriver. If you really want to keep it, solder them together."
"Re the Torx 15 screwdriver in the UK - I found that a standard long-handled (8 inch?) 3mm blade flat screwdriver does a nice job of opening up Torx 15 screws. I bought mine from Farnell. They have a huge range of tools - I got the smaller Torx drivers for Powerbooks there too, at a very reasonable price."
"In Denmark, the hardware stores of Johannes Fog carry Torx drivers, including T15s. There's a store in Kongens Lyngby, north of Copenhagen, at Nørgaardsvej 3, tel. 45 87 10 01."
"The tool to open the case is an Xcelite #XTD-15XL. This is a #15 Torx driver on an 11 inch shaft. It costs about $5. If you don't want to buy the tool, borrow one, then replace the Torx screws with Phillips ones."
"The major problem isn't finding Torx bits, but finding a Torx screwdriver long enough to fit in the handle. I had a Torx bit lying around (the kind you're supposed to use with a cordless drill). Obviously, this would not be long enough. It turns out that the barrel of an ordinary Bic 'stic' pen is just long enough to reach into the handle of the Mac, and it holds the bit firmly enough that it can be used as a screwdriver. If the screw is a bit stubborn, you can hold the pen barrel with a pair of vice grips."
"You can get Torx screws out with Allen wrenches, but it's really not a good idea. I mutilated a bunch of screws and cheap wrenches this way. Since General Motors uses a lot of Torx screws, the appropriate driver is widely available."
"In the US, Sears sells a Craftsman Professional T-15 Torx screwdriver for $5 that is long enough to open the SE case. Its part number is 47431."
"If you have a hard time finding a long Torx driver you can use a 3/32 inch Allen wrench. It will fit in the Torx head screws. This tool is what I was provided with when I ordered a "compact Mac toolkit" several years ago. A long T-shaped 3/32 inch Allen wrench should be available at hardware stores. You can also use an Xcelite #R188 screwdriver to remove these screws. It is a thin-bladed standard screwdriver, which you should be able to get at most electronics stores. The blade fits the screws perfectly, although slips are sometimes unavoidable. Try this if all else fails."
"For all the Torx you will ever need visit http://www.wihatools.com/indexes/indxtorx.htm. I use their 'System 6' handle with an extension and a Torx 107 series blade."
"I use a combination of one magnetic Torx bit, two magnetic hex bit holders (the sort of thing you use in power drills), and one screwdriver handle to fit all the above. It's kinda Heath Robinson but it works a treat. You have to be careful the drill thingies have a small enough diameter to fit in the gaps in the case, mind."