My only tank smaller than 300cf is the 150 on my custom built cart (post#4 this thread) - and the ONLY reason for that is it's the largest 7" diameter bottle offered, and keeps my cart a bit narrower AND lighter than it woulda been.
Check the pics, you'll see holders for torch, MIG pliers, and nozzle gel (with room for a battery drill beside it) - The pics are decent rez, you can zoom in quite a bit for more details.
By building my own I tic'd about a DOZEN more boxes than ANYTHING I could buy readymade. The upper frame acts as a "roll cage" to protect the gauges, it has a lifting eye that's adjustable in 2 axes so it can be lifted LEVEL regardless of how full the bottle is, the top crossbar lets me tilt it back to be its own hand truck (in that position it runs on 13" no-flat tires) - there's a padded angle iron support for the welder (the wire door side was carefully designed to allow access WITHOUT removing the welder) -
There's a built in hitch for towing with either golf cart or riding mower - in that mode the smaller (5" dual locking casters) get retracted and locked UP with barrel bolts so they don't drag on uneven ground - the bottle gets locked in place with a band of 14 ga. steel (ends have coupler nuts to cinch the bottle in place) - there's a top holddown for the welder so it doesn't slide out when cart is tipped for "hand truck" mode -
Only downside is time; probably took me about a month of messing around, but I grin every time I use it
... Steve