Here's what I cam up with: a variation on SGNerf's pvc pumpgrip.
As you can see, it looks kinda unwieldy, but it works fine and my small hands can grip it fine. It also fits over a stock LS with removed bipods just fine (SGNerf can fit thinwall pvc over the bipods; thinwall pvc is very uncommon in my area). The pvc arm bars can (supposedly) withstand spring tensions of 14kg and upward. Other than that, I can churn these out for $15.
The JSI (NerfNerds-style inspired) pumpgrip that I use on my own LS, however, is slightly better in my own opinion.
As you can see, it requires a bit of modification to the front of the LS; the bipods CANNOT be reattached, and there is also a gaping hole in the shell (which can be patched). The upside is that the pvc is the same width as the LS shell, and fits quite nicely. Also, the metal brackets are very strong and can stand up to pretty high spring force. I can make these for about $20 - $25 (depending on where I can source parts).
Both designs use a #8 threaded bolt with nylon spacers to connect to the bolt sled.
So, in conclusion:
- SGNerf PVC LS Pumpgrip: $15
- JSI/NN PVC/metal Pumpgrip: $20-25