Yes, that is fine.
Some things to keep in mind:
- Adhesive tape squashes down over time. Better to use rigid materials. Doesn't have to be fancy, even paper will get you there -- you're probably not going to need above Class A fireproof/temperature ratings for a hand-wound transformer. (Or if you do, well, choose materials accordingly!)
- The effective gap is slightly less than twice the geometric gap, because the cross-section spreads out in the vicinity of the gap (fringing effect). That is, the gap area is effectively wider, which is equivalent to it being thinner. So plan to measure the inductance and try a few sizes of shims to get it on target.
- The same effect also increases apparent saturation current: more leakage loops back around the windings themselves, or within a core half, thus reducing the field strength within the core itself. This isn't significant for small gaps
- Gap size is relative to the minimum aspect of the core. So, compared to the diameter of the center peg (for ER/ETD), for example; but the side legs are narrower, and gap size is relative to the narrower axis. So expect side gaps to count for less, for this reason.
- Fringing field also means external field. Roughly speaking, the gap acts like a wire loop of the same dimensions, carrying whatever amp-turns are going through the core. Nearby metal or wiring (again, nearness is relative to gap height) experiences substantial induction (eddy currents), which can lead to interference or heating. This causes increased copper loss for solid wire near the gap (litz is preferred over solid wire!), and can cause EMI problems as well.
Tim