It has been elsewhere posted that a leather strap works well and I agree. You can get a leather strap in your price range.

I have a nylon web strap (basically a seatbelt strap =P) and a leather strap and I can say for sure the leather is more comfortable.

Obviously a soft leather will be more comfortable than stiff leather. But either one will 'take your shape' over time. You'll want one at least a couple of inches wide at the shoulder.

As far as using on both guitars, you might have to adjust it when switching. Unless you Seagull has a strap button on the body, you'll be tying the strap to the headstock, so it'll need to be longer than you would use with your Squire. You really need two.

My 2 cents. :)