I made a custom forend, mine is just under 2" wide and slightly longer than a 9" standard forend. I think the GSI varminter is about 13". I have another zebrawood forend I haven't finished and it's about 11". Longer forends have more chance to contact the barrel and influence accuracy, so if it was me building a varminter, I'd make a short, wide forend and add weight if necessary, rather than build a long one. Search on "zebrawood" and you'll see my stock.
I made a few mistakes along the way, so if you're making your own, I'll offer the following advice:
1) Square up the forend on a table saw and then mark the center line.
2) Drill a 1/4" hole for the forend screw on the center line. Drill the hole a smidge further forward than you need, this way you can adjust forend tension by carefully removing material underneath the forend spacer. Don't countersink the hole just yet.
3) Put a 1 1/4" core box (or whatever size barrel you have) bit in your router and adjust the fence so you're on the center line. On your first pass with the router, make a very small cut. You'll be able to see if you're exactly on the center line because you'll pass directly over the hole you drilled. Route the cavity and flare the back end with sandpaper wrapped over a dowel.
4) Now you'll need to remove some material for the forend mounting screw. There are two operations: the flange on the boss sticks up, and the diameter of the boss is larger than 1/4". You can either chisel out the part where the flange on the forend is, or get lazy like the factory and drill a wide shallow hole. Next drill a 3/8" (I think) hole for the forend boss. Set the height so that you don't drill all the way through! If you do, you'll have a big sloppy hole for the 1/4" mounting screw. That's OK, but why not be neat?
5) Now you should be able to test fit your forend. Don't drill the forend spacer screws, just slap it on and get a friction fit. It should go on really tight if you drilled that hole a smidge further. Trace out the forend spacer on the wood using a sharp pencil so that you know where it goes. Now go ahead and drill the two mounting holes.
Since your forend is still rectangular, it's easy to take off a slight bit off under the forend spacer, just put it on the chop saw and take one hair at a time. My forend fits on snug and doesn't fall off, but I can remove it easily. For a varmint rifle, I'd make this fit on the loose side (but not falling off) and use the forend screw to adjust tension.
6) Shape the forend how you like. I used a round-over bit for the general shape, put some finger grooves with a smaller core box, a band saw to do the front taper, and then a long strip of sandpaper in either hand (60 grit) with a back and forth motion to even out the profile.
7) Countersink the forend screw. If you made a beefy varminter forend, you'll need a longer screw. Sand and finish as desired. Most people seem to use tung oil, but I went to 320 grit and used West Systems 207 epoxy. If you put the forend screw in before the epoxy, a small rim will form around the hole, which captivates the forend screw. Now when I remove the forend, I can't lose the screw.
