Try making a frame out of 100 mm wide material with a moisture content of 17% (i.e. typical joinery quality softwood). Place frame in a centrally heated environment and within a few weeks you are guaranteed to have 5mm gaps on the inside (but not the outside) of all the mitres. That's because the wood is now narrower and the angles are therefore no longer 45 degs. You can either take the frame apart and re-cut the mitres or use a dryer material in the first place, but they can never be clamped tight. Ever.