A Nature Reserve in the middle of your base?

1. Pips
2. Seeds
3. A base designed to fit it (sorry, you might have to restart to do it right).

The basic setup you're looking at is a vertical shaft through your base, consisting of 5 block high and 8 block wide modules.

First and eigth column consists of two doors and two blocks of wall above. In front of the doors, you have to leave one block of natural soil, then leave a block empty (fill it with an airflow tile as soon as you research it), with the two center blocks consisting of a ladder and a pole to allow your duplicants to climb up and down.

Every single one of those modules has a total of 24 blocks, meaning that if you stack 5 of them on top of each other, you're looking at a total of 24*5=120 plus 4*2 blocks for the blocks between them. So we're 8 blocks too large. Just fill 8 blocks at the top of the rooms with more solid blocks and you're down to 118.

Now all you need is some Pips to bury some seeds in the natural blocks you left. You can easily get them from the printing pod, sometimes you get pip eggs or young pips offered. Take them (do NOT capture and release them! That makes them tame and you'll have to feed them), just open the doors from the printing pod to the shaft you want them to seed, put a storage bin with seeds in them into the nature-reserve-to-be and wait for the Pips to plant seeds.

The shaft looks something like this:

My suggestion is to put mostly Thimble Reeds, Oxyferns and other seeds into the storage bin that you can get some benefit out of (and that grow in the base).

Now all that's missing is a park sign and you can easily cash in a +6 morale bonus without having to break the bank.

As you can see, the layout of the base will probably not allow you to have a natural block in front of every door, but all you need is 4 such spots in front of a total of 10 doors.