Fit a paper box lid to its base
Fit the lid to the assembled box, allowing for folded corners and any decorative covering.
Measure the finished base
A lid should slide over a box without binding or wobbling excessively. Measure the outside of the folded, fully dried base rather than relying only on its original drawing. Glued tabs and overlapping cardstock add thickness at the corners. Check the top edge near both ends as well as at the middle.
Record the greatest length and width. For a small box, adding a total of 1 to 2 millimeters in each direction can be a starting allowance. It is a trial measurement, not a universal rule. Soft cardstock, uneven corners or an added paper covering may require more clearance. Make the test from the same stock you intend to use for the finished lid.
Build a shallow trial lid
Draw the lid's inside rectangle using those measurements. Add a border for the sides, perhaps 1.5 centimeters, or about ⅝ inch, all around. Cut the outer shape, score the four folds and create corner tabs. Fold it together with small clips before gluing anything, then lower it gently onto the box.
It should go on without force and lift away without pulling up the base. If one corner catches, look for a protruding tab or a lump of dried adhesive. Extra pressure will not solve that unevenness. When the whole lid is too tight, enlarge the central rectangle and make another sample instead of bending all four sides outward.
Include every surface layer
Paper wrapped around the base increases its outside dimensions. Paper lining the lid reduces the available space inside. Settle those layers before deciding that the fit is finished. Thick decoration or varnish on the surfaces that touch each other can also interfere with opening and usually serves little purpose there.
Glue the tabs only after the trial fit works. Let the lid dry separately so stray adhesive cannot bond it to the base. Check it once more with the intended contents inside; a tightly packed box may bow outward slightly. Keep the successful template with a note naming the cardstock. A change of material means another fitting test, even when the finished box is intended to look exactly the same size.
