yeah those tabs are going to break. I've done the same sort of glue job but using the fiber-carbon-plastic strips that are used to clamp things on pallets - works very well! I regret throwing out two or three of these covers years ago when the tabs broke - it's a pretty salvageable fix
Here are Mashek's notes on this BTW - my takeaway is get the plastics nice and hot before trying to clip them onto the o2, and I personally don't think I would attempt to take the scalp cover off - the tab will also break and needs the same sort of reinforced glue job, but here are his notes anyway:
The O2 plastics have always been somewhat fragile, but with age, they have gotten much
more so.
Do not even attempt this in a cold environment. In order to put in place the
O2 fan cover without breaking off the tab the following steps need to be performed:
1. Eject the CDROM before shutting down or with a straightened paperclip
through the eject hole.
2. Remove the blue CDROM face from the CDROM drive tray by releasing the tabs
on the bottom side and sliding each side.
3. Gently push the CDROM tray almost closed.
4. Remove the top scalp by releasing the tab which is under the blue shroud above
the power supply fan. Be very careful as this tab breaks very easily also. It only needs
to bend forward (towards the front of the machine less than 1/8th inch). At the same
time use a longer fingernail to lift up on the back of the scalp plastic piece.
5. Remove the 2 or 3 large Philips screws and lift the shroud about 1/4th inch.
6. Put the bottom tabs of the fan cover in the slots of the power supply and tip and hold
the fan cover up against the power supply while lowering the shroud back down
into place. The back of the shroud will be beyond the tab of the fan cover.
7. Replace the screws to hold down the shroud.
8. Put the scalp back on by first getting the front in place and gently pushing the back
down so that the tab will latch.
9. Gently pull the CDROM tray out and reattach the blue face cover.
This should work. Do it in a warm environment (like 80 degrees F) and be very gentle.