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Motedis sell two types of 20mm extrusion, the I-type and the B-type. I used the I-type, however it wouldn’t fit into the vertical holes in the Robotdigg metal corners without enlarging the slots with a Dremel, and I needed to tap the end hole M5 myself. So B-type verticals would probably be better, as long as the slots are not too wide for the wheels. Motedis only sells 2060 extrusion in I-type, therefore either the horizontal extrusions must be I-type, or you could use 6 x 2020 B-type extrusions for the lower horizontals instead of 2 x 2020 and 2 x 2060. Also note that the B- and I-type extrusions take different sorts of T-nuts. The B-type takes M4 T-nuts that rotate into place. The I-type take M3 or M4 T-nuts that can be pushed into the slot and have a spring-loaded ball to hold them in place.

You could use V-slot extrusion if you prefer – just check that your wheels are compatible.

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nebbian
To be honest, most people won't notice those errors. But I have, and it frustrates the living daylights out of me. Note, if you do notice that parts print larger in an area of the bed, there's pretty much no way to figure out what's wrong, without meticulously going over every measurement again.

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nebbian
Building a cartesian printer such as a CoreXY would easily allow much better accuracy, without spending weeks going around and around in circles trying to figure out why parts are printed at different sizes in different areas of the bed.