Robotron on 7800 is one of the best games for the system!
However, you do need to use some sort of dual controller setup as mentioned several times above.
Edladdin's Super 78 is incredible but on the cheaper side of making things work, as others have suggested, two CX-40s would work well. If you are able also consider two CX-24s coupled or bound together; this works really well for playing Robotron 2084 on the 7800.
And though it is not as free as making your own -- which isn't too hard if you are resourceful and determined to play Robotron the right way -- there are already options here at AtariAge made and sold by amiman99 for CX-24 and CX-40 couplers (in addition to his CX-52 controller coupler). amiman99's CX-24 and CX-52 couplers work great; both are very sturdy and comfortable.
Edited by Fingolfin, Thu May 10, 2018 8:28 AM.