I have an m12 and my son has an mts. Currently have a puffed reedy in my son's, and I'm using the double a tray in my m12. Looking for some wallet friendly options for the future

A simple LiPo does not cost much, look at Hobbyking for cheap priced LiPo's but even at a local hobbyshop you should get something that fits for a reasonable price. Or Go back to NiMh and spend a little bit more for Eneloop batteries (low self discharge)

If you can, get a LiFe pack like the one Sanwa sells for the M12 that will self-discharge less and tolerate lower voltages. The Sanwa is more expensive, if this pack comes back into stock on hobbyking might suit and should fit.

The Sanwa pack measures approximately 95x32x17mm. If you go for a different battery, you can attach a servo connector onto any 2S pack that will fit into those dimensions.

I just tested the Radiomaster 5000mAh li-ion pack in the M12S and it is just a bit too big. The battery cover can close, but it bulges in the centre.