I would suggest using 1HP blanks if you dont want little exposed gaps as there are no particulary usable modules that I can think of, which are that small.

Here are some honourable meantions for 1HP blanks

https://www.modulargrid.net/e/fully-wired-electronics-1hp-blanking-panel (Fully Wired Elecronics Store - https://www.fullywiredelectronics.com/product-page/1hp-blanking-panel-3u)

https://www.modulargrid.net/e/make-noise-1hp-blank (Thomann - https://www.thomann.de/de/make_noise_1hp_blank.htm)

https://www.modulargrid.net/e/doepfer-a-100b1 (Thomann - https://www.thomann.de/de/doepfer_a_100_b1.htm?ref=search_complete_a%20100b1_suggestion_products%20-%20Doepfer%20A-100%20B1_Doepfer%20A-100%20B1_0)

That said gaps are of course useful as venting