<p>I have <a href="https://ultimaker.com/en/products/ultimaker-2-plus" rel="noreferrer" title="Ultimaker 2+">Ultimaker 2+ 3D Printer</a> and I need to print a piece that doesn't fit within the build volume of the printer. Even though it would fit I'd still need to print it in two parts because I'll need to fit some equipment inside. I could use glue to put it together, but I'll need to remove the equipment later.</p>

<p>So I'm trying to find some feasible solutions how to attach/snap it together. The wall thickness is currently 3mm.</p>

<p><a href="https://i.stack.imgur.com/FFckZ.png" rel="noreferrer" title="Cut Plane"><img src="https://i.stack.imgur.com/FFckZ.png" alt="Cut Plane" title="Cut Plane"></a></p>

<p>Plane for cutting the part.</p>

<p><a href="https://i.stack.imgur.com/L2hhV.png" rel="noreferrer" title="Cut Cross Section"><img src="https://i.stack.imgur.com/L2hhV.png" alt="Cross Section where I split the part" title="Cut Cross Section"></a></p>

<p>Cut cross section.</p>

If the equipment has to be removable, then there's no point in trying to make a one-piece object in the first place.  So it looks like you have two problems. The first is to decide what's the best way to split your container to facilitate both putting the equipment inside & removing it; the second is how to latch the two together.  I can't answer the first since you haven't shown us the equipment.
As to the second: there are a number of plans for spring-latching connectors (such as used with straps, backpack covers, etc) on thingiverse.com.  If you have no constraints on the exterior of your container, I would just merge the latching connectors into the container wall (e.g. with meshmixer) .