It's not impossible to do it on your own, but unlike other countries, Africa has no sita/aftn and no email. So permits must be done thru fax or thru phone calls. Expect phone lines to be rough and difficult also, unlike in Europe or the US.
The first three companies I've mentioned are not that good in African countires, besides those companies just use Hadid/UAS/Palm/ASM Flight Support/Jetex for their african permits. I've checked on their websites, average charge for Overfly permit processing is around 50USD/permit and for landing permits 100USD, plus the Navigational Fee, which would not cost much as you're only flying a light acft.
You can check with those Flt Support companies regarding customs/immigration, yellow fever requirements (common in africa), airport operating hours, approximate handling charges and landing fees, and fuel price at your destinations etc. I think all of their pages have forms where you can ask your questions and you can expect an answer in 5-10mins, all of the above companies are 24/7 and are quick to respond, according to my experience.