<p>I want to use magnets to hold the lid of a box down tight enough to keep it relatively airtight (along with a rubber seal etc.), but I am not sure what strength of magnet to use, that will still allow it to be opened without causing damage either by having to be pried open or by crushing the print layers. I cannot seem to find any guide to how magnets are used for 3d printing at various strengths and I cannot afford to buy too many types that I am not then going to use.</p>
<p>Any help that you can provide will be greatly appreciated.</p>

I had a similar experience using magnets to hold two plates together. Currently also building a device (3d-printed) that clamps together with magnets. For both of these scenarios, I typically start by looking at what size and force you need. I would look into maybe 3 options of different magnets to start with. It could be in the range of 0.6 lbs, 4lbs and 6lbs. Do you know how much the "opening force" would be? As in what would keep it opening? If its substantial, I could see more the 6lbs but this is a very very very rough estimate that honestly would be best supported with trying a few ranges of magnets first. Try McMasterCarr if you're in the US. They have affordable sized magnets that I've worked with, ranging from different sizes, thicknesses and magnetic forces.