func (h *ProposalHandler) PrepareProposal() sdk.PrepareProposalHandler {
    return func(ctx sdk.Context, req *abci.RequestPrepareProposal) (*abci.ResponsePrepareProposal, error) {
        // 3. inject a "fake" tx into the proposal s.t. validators can decode, verify the checkpoint
        injectedCkpt := &ckpttypes.MsgInjectedCheckpoint{
            Ckpt:               ckpt,
            ExtendedCommitInfo: &req.LocalLastCommit,
        }
        injectedVoteExtTx, err := h.buildInjectedTxBytes(injectedCkpt)
        if err != nil {
            return nil, fmt.Errorf("failed to encode vote extensions into a special tx: %w", err)
        }
        proposalTxs = slices.Insert(proposalTxs, defaultInjectedTxIndex, [][]byte{injectedVoteExtTx}...)

        return &abci.ResponsePrepareProposal{
            Txs: proposalTxs,
        }, nil
    }
}