subwoofer connection to unfriendly amp

I have a REL B-3 sub. The REL cable is terminated with banana connectors on the amplifier end which works fine for connecting to my VAC PA100 output terminals. I would also like to be able to connect the sub to my VAC Phi 110. I like to switch between the two different amplifiers and also would like to be able to utilize the sub with either amp without a total rewire each time.

The Phi 110 has ouput terminals whereby you tighten both +/- leads at the same time via a large knurled knob that is secured to a threaded post between the terminals and presses inward against a plastic bridge. There is no provision for a banana connector.

Since the REL cable only uses three terminals, even if I switched to spade connectors, I would not be able to tighten the speaker wire on one channel due to one terminal possessing two wires and the other only one wire. Plus it is difficult just as it is, to hold the speaker cables in place and tighten the knurled knob.

I was thinking of making an adapter box with speaker terminals that would accept the banana terminals mounted into a small box. Then run some decent wires out and use eyelet terminals on the amplifier posts to prevent them falling off. I would also add one dummy wire so the the amplifier terminal will tighten evenly.

So the questions are: Where is the best place to find the parts to do this? Is there a better way to accomplish the subwoofer changover between the two amplifiers that I have not considered?