The welding table I use when at home is an interpretation of a folding table that I saw on the Miller website some time ago. It works well enough but one of the problems with it is that it is not here.
One thing I do have here is a battered Gumtree 'Black and Decker Workmate 600', which is sturdier (and so more useful) than I originally thought it would be. Welding on it however is not really practical because it has a wooden top and is a little low.

I've made a deal that means soon I will need a welding table so I decided a false top for it may be the answer.
The top is a sheet of Al, 5mm thick, 600x410mm (because that is what a local sheet metal shop had in the off cut bin)
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The frame is 50x50 RHS underneath with 50x50 angle around the top so that there is a flange that I can clamp to if needed
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To get the 100mm extra I wanted, I've used some 20x20 RHS I had from another task to space things up. It sits solidly on those bearers so there is no rocking.
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The workmate is 700mm long and I was disappointed that the sheet would not quite cover the whole top but then discovered that the holes in the end of the workmate top (originally for the B&D clamping system) are just a nice size to both hold the chipping hammer and the electrode holder. The Al sheet I've left loose as sometimes you need to move things around but it could be secured*. I expect that this arrangement will work well enough but if necessary I can add additional insulation or shielding. Once it has been used and there are no changes required, I may even find that can of paint and make it look pretty.

*The sheet will get hot and so expand, so I'd use a standard bolt in one corner and then on the others, larger than normal clearance holes in the sheet with mudguard washers and a sleeve on the bolt so the joint can move when necessary while holding the sheet down. If it is bolted down rigidly, it may buckle/ warp.

Michael