If you store a fixed amount of data regularly, the "filesystem" can shrink into a single number, keeping the number of stored records. You can add a timestamp and serial number to the records, still having them constant length.
For PC readibility of CF, it is enough to create one big file spanning over the whole card. You can create it on PC, finding the position of its first sector and simply write into it. Alternatively, you can create multiple files with filesizes of multiple of cluster size; on a freshly formatted CF they will be allocated after each other.
You need to consider also that the CF sector size is 512 bytes, so you need a 512B buffer to store it when adding your 20-30B. Also, this is the minimum cluster size for FAT, so if implementing FAT and storing each record as a separate file you will have huge overhead here.
Human readability can be often a bonus, consider writing your data as ASCII.
Beware, some CFs today tend to be crappy - we came across a type which has incorrectly implemented the 8-bit mode... Also you need to have a quite long reset pulse for some types to work correctly. Some of the similar projects use MMC which are said to be easier interfaced to a mcu, but I have no experience with them.