The focusing screen from your film bodies would be bigger in size compared to the one on your K-m (or for that matter any current Pentax DSLR). You can of course cut down the screen by DIY but unless you can get the measurements and trimming correct, you run the chance of having the center focusing aid off-center. As you'll need to trim on all 4 sides, you'll also need to make a tab to hold that screen with a pair of tweezers.
My suggestion is to buy from
focusingscreen.com and save yourself the hassle. I've used several of their screens on different Pentax DSLRs and they are great and easy to install. The screens are machined down to size from stock screens. Best imo is the Nikon K3 screen, definitely brighter than the Canon screens and no issues exposure wise. Definitely cheaper than Katzeye and shipping is very fast.